It is known in the art of sonic position measurement systems to use an ultrasonic transmitter connected to the object whose position is to be detected. It is also known to have the sonic transmitter transmit a sonic pulse along a sound signal conductor which extends along the path which the object moves, the conductor having a predetermined sound propagation speed. The sonic pulse is sent in both directions along the conductor and is received by receivers at each end of the conductor, between which the object moves. Each receiver provides an electrical signal (indicative of when the pulse arrives at the respective receiver) to an evaluation circuit, which measures the time difference between the arrival of the pulse at the two receivers. From this time difference and the known speed of sound in the conductor, the position of the object between the receivers is determined. Such a system is described in European patent application EP 0,694,792 A1, published Jan. 31, 1996 filed Jul. 21, 1995, applicant K. A. Schmersal GmbH and Co. of Germany.
In an elevator application, the operation of an elevator depends on accurate and reliable position and speed information. This information is used to control the motion of the elevator as well as certain safety roles and operations, such as opening of the doors. Modern elevators use a high-resolution position information from a source such as an incremental encoder on a motor to control the speed profile. However, for safety-relevant functions they rely on additional discrete switches in the hoistway.
One way to implement a high-resolution measurement of the elevator car's position in the hoistway is to use the sonic position sensor discussed hereinbefore. In that case, the measurement ultrasonic pulse is launched along a wire from the car to the receivers at the bottom and top of the hoistway. From the difference of the reception times, the position of the car is calculated.
However, such a system has numerous disadvantages. In particular, if the length of the signal conductor changes due to temperature effects it induces inaccuracies into the measurements of the location. Furthermore, using the difference between the reception times provides only one channel of position information. Thus, an accidental displacement of a receiver or any other change in length of the signal path results in erroneous measurements without the ability of being detected from the signal information. Further, inaccuracies induced by temperature-dependent variations in the signal speed are fully transferred to the position measurements. Accordingly, it is desirable to overcome these drawbacks for an elevator application.